Electrolytic Cell Comprising Multilayer Expanded Metal

ABSTRACT

The present invention relates to an electrolysis cell comprising sheet-like anodes and cathodes which are separated from one another by means of separators and which are arranged in a cell trough or in a plurality of electrode frames clamped to one another and are connected in an electrically monopolar or bipolar manner, wherein the cathodes and/or anodes are in the form of multilayer expanded metal electrodes which consist of at least two expanded metal layers which are in contact with one another via internal resistance zones and through which the electrolyte solutions flow in the longitudinal direction, and to its use for the cathodic reduction or the anodic oxidation of organic or inorganic compounds.

The present invention relates to an electrolysis cell for the electrolytic treatment of process solutions and wastewaters. Preferred applications are cathodic reductions and anodic oxidations of inorganic and organic compounds and the cathodic deposition of metals in the case of low residual contents. The novel electrolysis cell has a large specific electrode surface area, toward which flow is optimum for achieving good mass transfer. It is therefore particularly suitable for effectively carrying out those cathodic and/or anodic electrochemical processes which take place under mass transfer control and for which high specific electrode surface areas and good mass transfer conditions are important preconditions for high current efficiencies at low specific electric energy consumptions.

Electrochemical processes and electrolysis cells suitable for this purpose have already been described in the literature.

Thus, EP 0 436 146 A1 discloses a process for the electrochemical regeneration of chromosulfuric acid, which process is carried out in an electrolysis cell which is formed from two tub-like metal half-shells with an ion exchange membrane arranged inbetween.

EP 0 573 743 A2 describes a process for the electrolytic decontamination or regeneration of a cyanide-containing aqueous solution, and an apparatus for carrying out this process. The electrolysis cell used for this purpose according to example 1 has sheet-like electrodes arranged parallel and a distance apart, the anode of coated titanium being effective on both sides and the cathodes arranged at the ends consisting of expanded metal lattices of copper-plated titanium.

An electrode arrangement for gas-forming electrolytic processes in membrane cells is described in WO 94/20649. This consists of lamellar expanded metal elements which are arranged one on top of the other and whose upper edges are angled backward for facilitating gas removal and rest against the ion exchange membrane at the front.

For carrying out mass transfer-controlled cathodic and/or anodic electrochemical processes in a manner which is as complete and economical as possible, it is necessary both to provide a sufficiently large electrochemically effective electrode surface area and to realize advantageous hydrodynamic conditions for optimum mass transfer from and to the electrode surface. Often, however, an increase in the available specific surface area does not also simultaneously lead to an improvement in the mass transfer, generally due to poorer flow toward the larger electrode surface regions. This is often the case with the so-called three-dimensional electrodes, in particular in the case of fixed-bed particle loads and in the case of porous electrodes toward which the electrolyte flows on only one side and which have a large specific surface area, based on the area projection.

In addition to the possibly insufficient hydrodynamic flow toward the electrode surfaces, a disadvantageous potential distribution in three-dimensional electrodes is generally also responsible for the fact that the available electrode surface can be electrochemically utilized only in part. Even in a thin surface layer facing the opposite electrode, there is often such a large drop in current density that the surface regions further away cannot even participate in the electrolysis process. Thus, for example, WO 00/34184 describes an electrolysis cell having a so-called “open configuration”, in which the electrodes may be made of a plurality of layers of expanded metal which are in contact with one another. As a result of being in contact, there is no influence on the current density distribution between the individual layers. By far the greatest current input into the electrolyte takes place via the expanded metal layer arranged closest to the opposite electrode and falls off sharply toward the adjacent layers.

In order to avoid such a drop in current density with increasing distance from the opposite electrode, DE 36 40 020 proposed divided electrolysis cells having one anode and a plurality of planar liquid-permeable cathodes arranged so as to be electrically insulated from one another, the ohmic resistances increasing with increasing distance from the anode being compensated by higher applied cell voltages so that the current is uniformly distributed over the individual cathodes.

DE 40 07 297 proposes the same principle but with one cathode and a plurality of liquid-permeable anode plates with separate current supply. In these electrolysis cells, the electrolyte flows through the cathodes or anodes formed as liquid-permeable multiple electrodes in a direction perpendicular to their longitudinal dimension.

Although the desired increase in the surface area which is actually electrochemically effective in combination with uniform current distribution of the individual electrodes is achieved by means of these multielectrode electrolysis cells, this is also associated with a number of disadvantages. Thus, the apparatus required for the separate current supply to the individual electrodes and for an adapted current distribution is relatively complicated. The separate current supply requires either a plurality of controllable rectifiers or, in the case of only one rectifier, inclusion of different external resistances for regulating the electrolysis currents for the individual electrodes of the multielectrode electrolysis cell.

In the case of the current adaptation by means of different resistances, the required electrolysis current consumption is also greater since a part of the direct current applied is converted into heat outside the cell.

However, the electrolysis cells, too, are very complicated in terms of apparatus. Thus, the individual electrodes or electrodes combined to form groups, preferably consisting of metal wire nets, are to be arranged so as to be electrically insulated from one another and are to be provided with separate power connections.

Furthermore, the conditions for advantageous mass transfer to and from the electrodes are by no means to be regarded as optimum in the case of these multielectrode cells. Since the electrolyte flows through the electrodes transversely to their longitudinal dimension, large cross sections with through flow result. Very large amounts of circulating electrolytes are therefore also required in order to achieve a sufficiently high flow rate along the electrode surfaces toward which flow takes place transversely, and this only in the case of extremely short contact times between the electrolyte stream and the electrode surface.

Finally, owing to the required current supplies to the individual electrodes, it is virtually impossible in the case of such cells having multiple electrodes to combine a plurality of individual cells to form an industrial electrolyser having high current capacity by a bipolar electrical connection. In order to realize such industrial electrolysis cells of high current capacity based on the multielectrode cell, all that remains is the coupling of a large number of monopolar individual cells to be connected electrically parallel and having separate current supplies, which coupling requires a very complicated apparatus and is therefore often not economically acceptable.

It has now been found that the disadvantages described for the known cell constructions having large electrode surfaces, in particular for the multielectrode cells, can be substantially avoided with the use of a divided electrolysis cell which compared to multielectrode cells has a high specific electrode surface area and toward which the flow is optimum.

The present invention relates to an electrolysis cell comprising sheet-like anodes and cathodes which are separated from one another by means of separators and which are arranged in a cell trough or in a plurality of electrode frames clamped to one another and are connected in an electrically monopolar or bipolar manner, wherein the cathodes and/or anodes are in the form of multilayer expanded metal electrodes which consist of at least two expanded metal layers which are in contact with one another via internal resistance zones and through which the electrolyte solutions flow in the longitudinal direction.

Preferably, from 4 to 12 expanded metal layers are brought into contact with one another and with an electrode baseplate via internal resistance zones to form a multilayer expanded metal electrode. Intermediate layers of a porous electrode material can also be arranged between at least two expanded metal layers.

As a result of the expanded metal layers through which the electrolyte flows in their longitudinal direction, optimum preconditions are created for good mass transfer. For this purpose, preferably flow rates of at least 0.1 m/s, particularly preferably from 0.3 to 0.8 m/s, based on the mean free cross section of the expanded metal layers through which flow takes place, are established. The continuous deflection of flow on passage through the expanded metal layers, associated with an alternate constriction and expansion of the cross section through which flow takes place, results in microturbulence which permits very advantageous mass transfer from the electrolyte solution to the electrode surface, and vice versa. In the case of gas-evolving electrodes, there is moreover rapid removal of gas bubbles, so that only small stationary gas loads occur in the electrode interstices. The electrical resistance and hence the cell voltage increase only minimally.

In the case of a preferred expanded metal thickness of from 1.5 to 3 mm, relatively large distances result between the electrode baseplate and the expanded metal layer furthest away from said baseplate, for example 30 mm in the case of 10 to 12 layers. Depending on the electrical conductivity of the electrolyte solutions used, voltage differences in the electrolyte then occur at as low as between 0.5 and 2 V. This leads to a decrease in the current density with increasing distance from the opposite electrode. According to the invention, this drop in current density is compensated or at least buffered by internal resistance zones between the individual layers, so that a corresponding voltage difference between the electrode baseplate connected to the current supply and the adjacent expanded metal layers is also produced.

With the optional use of intermediate layers of porous electrode materials, the number of layers for achieving a desired surface factor and hence also the total thickness of the multilayer electrode can be substantially reduced. Such porous intermediate layers, preferably consisting of metal foams, knitted metal wire fabrics or nonwoven carbon fiber fabrics already have a relatively large internal surface area with a thickness of, preferably, from 1 to 2 mm and an intended large void volume of at least 70%. The surface factor (SF) as the ratio of the geometric surface to the surface area of the relevant expanded metal or intermediate layers is in the range of from 1.5 to 4 in the case of the expanded metal layers and between 10 and 20 in the case of the porous intermediate layers. For both layers, a large void volume of at least 70%, preferably from 80 to 90%, is an important precondition with regard to good permeability for the electrolysis current and the electrolyte flow.

The surface factor achievable by the multilayer expanded metal electrode should preferably be in the range between 5 and 100. The surface factors in the upper range can be realized virtually only by the use of porous intermediate layers.

The influence of the porous intermediate layers is to be explained for the example of a multilayer expanded metal electrode consisting of 9 expanded metal layers having a surface factor of 2 in each case. For the 9 layers, the resulting surface factor of the total multilayer expanded metal electrode, including the electrode baseplate, is 9=2+1=19, with a total thickness of the expanded metal packet of about 18 mm. In the case of only one porous intermediate layer with SF=14 and 2 expanded metal layers on an electrode baseplate, SF=2+14+2+1=19 gives a comparable surface factor with a substantially smaller total thickness of only about 6 mm.

Through the expanded metal layers which are arranged on both sides of the porous intermediate layer and through which flow takes place longitudinally, flow toward the porous intermediate layer takes place on both sides at a high flow rate. With the use of thin porous intermediate layers having a thickness of not more than 2 mm, introduction of the compounds to be reacted on the internal surface of the porous intermediate layers takes place from both sides over a short distance of not more than 1 mm. This makes it possible to make maximum use of these relatively large internal surfaces of such intermediate layers for the electrolysis process. The introduction of the compounds to be reacted and the removal of electrolysis products are effected with the electrolyte flow through the adjacent expanded metal layers.

The internal resistance zones to be provided according to the invention for compensating or buffering the decrease in current density can be formed by the contact resistances between the individual expanded metal and porous intermediate layers. The individual layers are pressed against one another and against the electrode baseplate only by pressure means, for example comprising plastic fins or plastic screws. Particularly in the case of the passage of current from an expanded metal layer to a porous intermediate layer, internal resistance zones which may lead to voltage differences of from 0.1 to 0.5 V are built up by the measured contact resistances. Between every two expanded metal layers, the contact resistances are generally lower. If required, the contact resistance can be increased to meet the requirements by reducing the contact area by partial covering with nonconductive materials, for example thin woven fabric inlays.

According to a further preferred feature of the invention, the internal resistance zones are formed by separating individual expanded metal and/or intermediate layers, or a plurality thereof, by spacers comprising electrically nonconductive material. The current transport from layer to layer is effected via contact areas preferably arranged laterally or above and below. Internal resistance zones are built up by virtue of the fact that the electrolysis current has to pass through the individual expanded metal layers in their longitudinal or transverse direction. The electrolysis current therefore flows through the multilayer expanded metal electrode in a meandering manner, the current decreasing from layer to layer according to the effective surface area and the real current density. The relevant expanded metal layers can thereby be welded to one another at the contact points in order to achieve good contact which is stable in the long term. It is also possible to carry out a substantial adaptation to the voltage drop in the electrolyte by connecting a different number of mutually short-circuited expanded metal layers electrically in series and thus adapting the voltage drops from layer to layer so that the decreasing current within the multilayer expanded metal electrode is compensated by increasing resistances.

Such compensation of the current density gradient which is as complete as possible is, however, absolutely essential only in those applications in which, in the interests of a high current efficiency, the redox potential must not exceed or fall below a certain value. In most application methods, there is a large current density range in which no unacceptably large reduction in current efficiency occurs.

According to a further preferred feature of the invention, a current density gradient present within the multilayer expanded metal cathode may even be deliberately utilized for achieving high current efficiencies and reaction yields. For this purpose, within an electrolysis cell, a plurality of expanded metal segments separated by porous intermediate layers is provided with different current densities by separate feed lines and discharge lines for the electrolyte solutions. The hydrodynamic connection is effected in a manner such that the electrolyte solution flows first through the segment closest to the opposite electrode and having the higher current density and then through the segment further away from the opposite electrode and having the lower current density. As a result, the main reaction, in which the starting materials are still present in relatively high concentration, is carried out in the expanded metal segment having the higher current density. If the starting materials are then present in low concentration, the reaction is completed in the expanded metal segment having the low current density. The same effect is therefore achieved as with a cascade of a plurality of electrolysis cells having different current densities, in which the main reaction takes place in the electrolysis cell having a high current density while the subsequent reaction is carried out in the electrolysis cell having a lower current density. However, the procedure according to the invention has the major advantage that no second electrolysis cell is required for the subsequent reaction. The space-time yield achievable is therefore also substantially higher. Moreover, there is the advantage that the hydrodynamic coupling of the two electrolyte streams can be effected by virtually the fact that the electrolyte solution can be transferred by means of an adjustable pressure difference from the segment of high current density through the porous intermediate layer into the segment of low current density. The flow through the porous intermediate layer is thereby simultaneously further improved.

The expanded metal layers to be used according to the invention preferably consist of stainless steel, nickel, copper or valve metals coated by means of noble metals, noble metal oxides or doped diamond. Expanded metals comprising coated valve metals are used mainly for anodic oxidation processes. Thus, very high anode potentials, as required, for example, for effective oxidative degradation of pollutants, can be achieved on expanded metals comprising niobium, coated on both sides with doped diamond, also at the desired relatively low current densities.

Instead of individual expanded metal layers or all expanded metal layers, it is also possible to use layers of woven metal wire fabrics. However, expanded metals are preferred because they have a more advantageous opening behavior for through flow in the longitudinal direction and also have a greater mechanical stability.

Particularly in the case of large required current capacities in the kA range, bipolar cells having multilayer expanded metal cathodes are preferably to be used. An advantageous embodiment of such a bipolar cell construction is shown schematically in FIG. 1. FIG. 1 a shows three bipolar cell units as a section through the electrochemically effective regions. FIG. 1 b shows a cross section through these three individual bipolar cells, likewise in the electrochemically effective region. The anode plates 1 and the cathode baseplates 2 are arranged on both sides of the bipolar electrode base body 3 comprising plastic. Integrated in the electrode base body are the cooling ducts 5 and the inlets and outlets for anolyte 6, 7, catholyte 8, 9 and the cooling medium 10, 11. In the case shown, the cooling duct is on the anode side, with the result that the anode plate present there is directly cooled. Cooling on the cathode side and cooling on both sides are also possible in principle. Resting on the anode baseplates and having an electrically conductive connection therewith are the multilayer expanded metal cathodes. Four expanded metal layers 12 in each case and a porous intermediate layer in each case, having a large internal surface area 13, are shown by way of example. The multilayer expanded metal cathodes are laterally bounded by a plastic cathode frame 4, which also ensures lateral pressure of the expanded metal layers against the cathode baseplates. The anode sealing frames 14, which bound and seal the anode spaces on the outside, are arranged on the anode plates. The ion exchange membranes 15, which separate the anode spaces from the cathode spaces, are clamped between the anode sealing frames and the cathode frames of the adjacent bipolar units. The anode spacers 16 and cathode spacers 17 comprising plastic serve for centering and holding the ion exchange membranes. The contact between the cathode baseplate and the anode plate of a bipolar unit is produced in this cell variant by contact rails 18 arranged on both sides outside the electrode base body.

FIGS. 2 to 5 show a plurality of variants for the bipolar electrode plates, once again as a section through the electrochemically effective regions.

FIGS. 2 and 3 schematically show sectional diagrams of two bipolar units having multilayer expanded metal cathodes for a catholyte circulation. Apart from the arrangement of the feed and discharge connections and an additional cooling duct on the cathode side, the variant shown in FIG. 2 corresponds in principle to the cell variant shown in FIG. 1. FIGS. 4 and 5 show the same bipolar units, but with separate expanded metal cathode segments for two separate catholyte circulations. In all cases, the multilayer expanded metal cathode consists of two segments in each case, having two expanded metal layers in each case and an intermediate layer of a porous electrode material having a large internal surface area.

In FIGS. 2 and 3, flow takes place in parallel through all four expanded metal layers. Flow toward the porous intermediate layer takes place from both sides, so that the mass transfer is only a short distance of not more than half the thickness of the intermediate layer to the interior thereof.

In FIGS. 3 and 5, the intermediate layer simultaneously forms the partition between the expanded metal segment close to the anode and having a high current density and the segment remote from the electrode and having a lower current density.

In FIGS. 2 and 4, the cathode baseplate is solid like the anode plate, except that passages are made for the supply and removal of the electrolyte solutions. It is therefore also possible, for both electrodes, to utilize the back spaces of the electrodes for the arrangement of cooling ducts for internal heat removal. In FIGS. 3 and 5, the cathode baseplate provided with the current supply is already equipped with an expanded metal layer, while the cathode back space here is filled with the catholyte. Consequently, only the anode back space can be utilized for the arrangement of a cooling duct.

FIGS. 2 to 5 do not show the manner in which the anode plate is in contact with the cathode plate of the respective bipolar unit. This can, as shown in FIG. 1, be effected by contact rails mounted on both sides outside the electrode base body. However, contact elements which make contact between the two electrode plates as a result of the pressure on assembly can also be mounted within the base body.

FIG. 6 shows the process diagram of an electrolysis system comprising an electrolysis cell analogous to FIG. 4, having two expanded metal cathode segments toward which flow takes place separately. The two cathode segments separated by a porous intermediate layer of low permeability are included in separate catholyte circulations with integrated gas separation. In the 1st circulation B, consisting of a circulation pump and a circulation vessel having a gas separator, the electrolysis is carried out at the higher current density. In the analogously structured 2nd circulation C, the reaction is completed at lower current density. Both circulation systems are coupled hydrodynamically to one another in a manner such that metering is effected into the circulation of higher current density (metering station D) and the electrolyte removal is carried out from the circulation of lower current density E. The passage through the porous intermediate layer is controlled by the pressure difference established between the two circulations. The anolyte is transported by means of the metering station G through the anode spaces. In the gas separator H, the gas emerging at J is separated off and the anolyte emerges at I.

In the case of smaller and medium current capacities, a monopolar electrical connection of the multilayer expanded metal electrodes is preferred for the electrolysis cell according to the invention, it being possible to arrange the expanded metal layers or the porous intermediate layers in a planar or cylindrical manner. In the case of the cylindrical arrangement, according to a further feature of the invention, a continuous expanded metal layer is wound in the form of a spiral and the beginning of the spiral is connected in an electrically conductive manner to the outer or inner tube of the cylindrical cell, which tube is provided with the current supply. An internal resistance zone can be built up in a simple manner if a layer of insulating spacer material is likewise wound in a spiral manner together with the expanded metal layer, with the result that the individual expanded metal layers are electrically insulated from one another. The electrolysis current is thus forced to take the longer route along the spiral, so that voltage differences form between the various expanded metal layers.

In the case of the planar arrangement, the electrodes and separation systems can be arranged either in a cell trough or in a plurality of electrode frames clamped to one another.

FIG. 7 shows, by way of example, a preferred embodiment of such a monopolar electrolysis cell having planar electrodes which are arranged in electrode frames clamped to one another. It shows the cross section through the electrochemically active region of a monopolar electrolysis cell having in each case two multilayer expanded metal cathodes connected electrically in parallel and two plate anodes which are separated by cation exchange membranes and are arranged in three plastic frames clamped to one another (the clamping frame is not shown in the figure). The two anodes plate 1 and the two cathode plates 2 are provided with current supplies and connected electrically in parallel. The anode plates consist of titanium and are provided in the electrochemically effective region with an active layer, for example comprising Ir/Ti mixed oxide. The multilayer expanded metal cathodes, which are arranged in one cathode frame 4 each, are in contact with the cathode baseplates of stainless steel. The multilayer expanded metal cathodes consist of 10 expanded metal layers 12 each, which are combined individually or in groups and are separated from one another by plastic cathode spacers 17. The various expanded metal layers separated by spacers are in contact with one another laterally in a manner such that the electrolysis current flows through the multilayer expanded metal cathode in a meandering manner. While the electrolysis current decreases from layer to layer, the resistance of the expanded metal layers connected to one another increases in the direction of the anode to four times the value (the number of expanded metal layers connected to one another decreases from four to one). The cathode frames contain the inlets and outlets for the catholytes 8, 9. Flow through the expanded metal and spacer layers is from bottom to top in the longitudinal direction of the catholyte. The centrally arranged electrode base body 3 contains the inlets and outlets for the anolytes 6, 7. By means of overflow openings, the anolyte passes through the sealing frames and the anode plates into the anode spaces at the bottom and out again at the top together with the anode gases formed. These anode spaces are formed by the anode sealing frames 14. Plastic anode spacers 16 on which the ion exchange membranes 15 are positioned are inserted.

A plurality of such monopolar cell units can be arranged within a clamping frame. Where large current capacities are required, the major advantage of the electrolysis cells according to the invention is that a bipolar connection of a plurality of individual cells in the clamping frame is also possible. Only two edge electrodes having current supplies are then required, between which any number of individual bipolar cells, limited only by the available DC voltage, can be arranged.

FIGS. 1 to 7 show only some of the particularly advantageous variants of the electrolysis cell according to the invention and the connection thereof and make no claim at all to completeness.

The electrolysis cell according to the invention is particularly suitable for the following application methods.

The electrolysis cell according to the invention and having a multilayer expanded metal cathode is used for the complete or partial cathodic reduction of organic or inorganic compounds. Both galvanostatic and potentiostatic electrolysis can be effected. Organic compounds which contain, as functional groups, C—C double and triple bonds, aromatic C—C linkages, carbonyl groups, heterocarbonyl groups, aromatic CN linkages, nitro and nitroso groups, C-halogen single bonds, S—S bonds, N—N single and multiple bonds and other hetero atom-hetero atom bonds can be reduced. Such reactions can be carried out in protic solvents, such as, for example, water, alcohols, amines or carboxylic acids, as well as in a mixture with aprotic polar solvents, e.g. tetrahydrofuran. If organic compounds are used which are not soluble in the abovementioned solvents, they can, however, also be brought into solution without problems by means of surface-active substances, for example higher alcohols, as solvents or solvent additives. Furthermore, the electrolysis of suspensions is possible.

In general, the electrolysis according to the invention is carried out in the presence of an auxiliary electrolyte, as described, for example, in EP 0808 920 B1. Here too, advantages arise through the high specific surface area of the multilayer expanded metal cathode, since it is often possible to manage with a smaller amount of added mediator in order to achieve the same results.

The electrolysis cells according to the invention and having multilayer expanded metal cathodes are particularly suitable for the complete or partial reduction of natural and synthetic dyes, such as, for example, carotinoids, quinone dyes, e.g. carminic acid and 1,8-dihydroxyanthraquinone, madder dyes, indigoid dyes, e.g. indigo, indigotin and 6,6′-dibromoindigo, vat dyes and sulfur dyes, and for the reduction of nitro functionalities. The novel cells are also suitable for decolorizing process solutions and wastewaters in dye works.

The reduction of indigoid and vat dyes can be carried out both as an indirect electrolysis, as described in the publications DE 195 13 839 A1 and DE 100 10 060 A1, and without the addition of mediator. In the reduction of sulfur dyes, such as, for example, C.I. Sulfur Black 1, to the corresponding Leuco Sulfur Black 1 compound, both complete and partial reduction can be carried out, as described in EP 1 012 210 A1. Here, the cathodic reduction is preferably effected in an alkaline medium (pH 9 to 14).

When equipped with multilayer expanded metal anodes, the novel electrolysis cells can also be used for the complete or partial anodic oxidation of organic and/or inorganic compounds. In particular, they are suitable for the oxidative degradation of pollutants in process solutions and wastewaters. Multilayer expanded metal anodes consisting of niobium expanded metals coated on both sides with doped diamond can particularly advantageously be used. Here, once again a direct electrochemical reaction can be realized, as well as an indirect one by means of mediators or by means of the anodic formation of oxidizing agents, such as, for example, peroxodisulfate, hypochloride or radical anions, which complete the desired degradation of pollutants in a subsequent reaction. However, the high anode potentials required for the degradation process can often also be achieved in such oxidation reactions by means of platinum-coated expanded metals comprising titanium.

Finally, cells comprising multilayer expanded metal cathodes can also be advantageously used in metal deposition, in particular from very dilute solutions. Electrodes of the metal to be recovered can be used and are changed after being laden with an appropriate amount of metal. In this case, the use of a trough cell with suspended, easily changeable multilayer expanded metal cathodes is advantageous. In order to remove small amounts of metals, for example from wastewaters, it is also possible to employ inert cathode materials from which the deposited metals are dissolved away periodically with a suitable solvent. Porous intermediate layers which have a large internal surface area and by means of which very small residual metal contents can be achieved are particularly suitable for this purpose.

EXAMPLE 1 Reduction of Indigo

In the cell according to the invention, comprising a platinum-coated titanium electrode and a sandwich cathode consisting of two stainless steel expanded metal layers and a knitted wire fabric layer situated inbetween, having a total electrode area of 2130 cm², the reduction of indigo to leuco-indigo is carried out.

The anode space and cathode space are separated by a cation exchange membrane (Nafion® 424). The divided cell is introduced into a two-cycle electrolysis apparatus having a pump circulation. The flow rate within the cell should be 0.1 m/s. 3000 g of water with 230 g of 50% sodium hydroxide solution were used as the anolyte. The catholyte was prepared by introducing 48.5 g of indigo, calculated as 100%, in a mixture consisting of 5000 g of water, 85 g of EC mediator VE PEDF 120 from DyStar Textilfarben GmbH, Frankfurt am Main, Germany, and 80 g of 50% sodium hydroxide solution into the cathode circulation. For the electrolysis, a cell voltage of 1.8 V and a current of 1 A are applied. The electrolysis was carried out at a temperature of 55-60° C. with a current density of 4.7·10⁻⁴ A/cm². The reduction was complete after 2 F/mol with quantitative current efficiency.

EXAMPLE 2 Reduction of C.I. Sulfur Black 1

In the cell according to the invention, comprising a platinum-coated titanium electrode and a cathode consisting of three nickel expanded metal lattices, having a total area of 1050 cm², the reduction of C.I. Sulfur Black 1 to the corresponding leuco compound is carried out.

The anode space and cathode space are separated by a cation exchange membrane Nafion® 424). The divided cell is introduced into a two-cycle electrolysis apparatus having a pump circulation. The flow rate in the cathode space should be 0.1 m/s. 3000 g of water with 234.75 g of 50% sodium hydroxide solution were used as the anolyte, to which a further 518 g of 50% sodium hydroxide solution are added gradually in the course of the electrolysis.

The catholyte was prepared by introducing 100 g of a C.I. Sulfur Black 1 press cake from DyStar Textilfarben GmbH into a mixture consisting of 2000 g of water and 12 g of 50% sodium hydroxide solution and adding a further 4350 g of press cake in the course of the electrolysis.

The procedure according to the publication EP 1 012 2100 A1 is employed for the electrolysis. A current of 10 A (current density: 9.5·10⁻³ A/cm²) and a cell voltage of 7.0-4.3 V are applied. The reduction was complete after introduction of a charge quantity of 238.3 Ah. 4350 ml of a leuco sulfur black 1 solution having a concentration of reducing agent equivalents of 438 Ah, based on 1 kg of dry dye, were obtained.

EXAMPLE 3 Reduction of Nitrobenzene

In the cell according to the invention, comprising a platinum-coated titanium electrode and a cathode consisting of three nickel expanded metal lattices, having a total area of 1050 cm cm² (visible area: 100 cm²), a reduction of nitrobenzene to azobenzene is carried out.

The anode space and cathode space are separated by a cation exchange membrane (Nafion® 424). The divided cell is introduced into a two-cycle electrolysis apparatus having a pump circulation. The flow rate should be 0.1 m/s.

1000 ml of 70% ethanol with 25 g of sodium acetate and 100 g of nitrobenzene are used as the catholyte. 2000 ml of a concentrated sodium carbonate solution are used as the anolyte. The procedure according to the publication Zeitschrift für Elektrochemie [Journal for Electrochemistry] 1898; 5; pages 108-113, is employed for the electrolysis. A current of 10 A (total current density of 9.5·10⁻³ A/cm²) is applied. The reduction is complete after a charge quantity of 87.5 Ah. The resulting yellow alcoholic solution is removed from the cathode circulation, the excess ethanol is removed in vacuo, and the residue is taken up in water and extracted several times with ethyl acetate. The combined organic phases are dried over sodium sulfate, and the excess solvent is distilled off.

63 g of azobenzene (85% of theory) are obtained with a quantitative current efficiency.

EXAMPLE 4 Reduction of Indigo

In the cell according to the invention, comprising a platinum-coated titanium electrode and a sandwich cathode consisting of two stainless steel expanded metal layers and a knitted wire fabric layer situated inbetween, having a total electrode area of 2130 cm², the reduction of indigo to leuco-indigo is carried out.

The anode space and cathode space are separated by a cation exchange membrane (Nafion® 424). The divided cell is introduced into a two-cycle electrolysis apparatus having a pump circulation. The flow rate within the cell should be 0.1 m/s. 3000 g of water with 230 g of 50% sodium hydroxide solution were used as the anolyte. The catholyte was prepared by introducing 48.5 g of indigo, calculated as 100%, in a mixture consisting of 5000 g of water, 120 g of EC mediator

VE PEDF 120 from DyStar Textilfarben GmbH, Germany, and 80 g of 50% sodium hydroxide solution into the cathode circulation. For the electrolysis, a cell voltage of from 2.2 to 2.0 V and a current of 2 A are applied. The electrolysis was carried out at a temperature of 55-60° C. with a current density of 9.4·10⁻⁴ A/cm². The reduction was complete after 2 F/mol with quantitative current efficiency.

EXAMPLE 5 Reduction of C.I Vat Yellow 46

In the cell according to the invention, comprising a platinum-coated titanium electrode and a sandwich cathode consisting of two stainless steel expanded metal layers and a knitted wire fabric layer situated inbetween, having a total electrode area of 2130 cm², the reduction of C.I. Vat Yellow 46 to its leuco compound is carried out.

The anode space and cathode space are separated by a cation exchange membrane (Nafion® 424). The divided cell is introduced into a two-cycle electrolysis apparatus having a pump circulation. The flow rate within the cell should be 0.1 m/s. 3000 g of water with 230 g of 50% sodium hydroxide solution were used as the anolyte. The catholyte was prepared by introducing 48.6 g of commercial C.I. Vat Yellow 46, calculated as 100%, in a mixture consisting of 5000 g of water, 171 g of EC mediator VE PEDF 120 from DyStar Textilfarben GmbH, Germany, and 80 g of 50% sodium hydroxide solution into the cathode circulation. For the electrolysis, a cell voltage of from 2.2 to 2.0 V and a current of 2 A are applied. The electrolysis was carried out at a temperature of 55-60° C. with a current density of 9.4·10⁻⁴ A/cm². The reduction was complete after 4.61 F/mol with a current efficiency of 87%.

EXAMPLE 6 Reduction of C.I Vat Green 1

In the cell according to the invention, comprising a platinum-coated titanium electrode and a sandwich cathode consisting of two stainless steel expanded metal layers and a knitted wire fabric layer situated inbetween, having a total electrode area of 2130 cm², the reduction of C.I. Vat Green 1 to its leuco compound is carried out.

The anode space and cathode space are separated by a cation exchange membrane (Nafione® 424). The divided cell is introduced into a two-cycle electrolysis apparatus having a pump circulation. The flow rate within the cell should be 0.1 m/s. 3000 g of water with 230 g of 50% sodium hydroxide solution were used as the anolyte. The catholyte was prepared by introducing 50 g of commercial C.I. Vat Green, calculated as 100%, in a mixture consisting of 5000 g of water, 171 g of EC mediator VE PEDF 120 from DyStar Textilfarben GmbH, Germany, and 80 g of 50% sodium hydroxide solution into the cathode circulation. For the electrolysis, a cell voltage of from 1.6 to 2.0 V and a current of 0.5 A are applied. The electrolysis was carried out at a temperature of 55-60° C. with a current density of 2.35·10⁻⁴ A/cm². The reduction was complete after 2.08 F/mol with quantitative current efficiency.

EXAMPLE 7 Reduction of C.I Vat Red 10

In the cell according to the invention, comprising a platinum-coated titanium electrode and a sandwich cathode consisting of two stainless steel expanded metal layers and a knitted wire fabric layer situated inbetween, having a total electrode area of 2130 cm², the reduction of C.I. Vat Red 10 to its leuco compound is carried out.

The anode space and cathode space are separated by a cation exchange membrane (Nafion® 424). The divided cell is introduced into a two-cycle electrolysis apparatus having a pump circulation. The flow rate within the cell should be 0.1 m/s. 3000 g of water with 230 g of 50% sodium hydroxide solution were used as the anolyte. The catholyte was prepared by introducing 50 g of commercial C.I. Vat Red 10, calculated as 100%, in a mixture consisting of 5000 g of water, 171 g of EC mediator VE PEDF 120 from DyStar Textilfarben GmbH, Germany, and 80 g of 50% sodium hydroxide solution into the cathode circulation. For the electrolysis, a cell voltage of from 2.4 to 2.8 V and a current of 2 A are applied. The electrolysis was carried out at a temperature of 55-60° C. with a current density of 9.4·10⁻⁴ A/cm². The reduction was complete after 4.51 F/mol with a current efficiency of 88.7%.

EXAMPLE 8 Reduction of C.I Vat Blue 6

In the cell according to the invention, comprising a platinum-coated titanium electrode and a sandwich cathode consisting of two stainless steel expanded metal layers and a knitted wire fabric layer situated inbetween, having a total electrode area of 2130 cm², the reduction of C.I. Vat Blue 6 to its leuco compound is carried out. The anode space and cathode space are separated by a cation exchange membrane (Nafion® 424). The divided cell is introduced into a two-cycle electrolysis apparatus having a pump circulation. The flow rate within the cell should be 0.1 m/s.

3000 g of water with 230 g of 50% sodium hydroxide solution were used as the anolyte. The catholyte was prepared by introducing 50 g of commercial C.I. Vat Blue 6, calculated as 100%, in a mixture consisting of 5000 g of water, 171 g of EC mediator VE PEDF 120 from DyStar Textilfarben GmbH, Germany, and 80 g of 50% sodium hydroxide solution into the cathode circulation. For the electrolysis, a cell voltage of 2.0 V and a current of 1 A are applied. The electrolysis was carried out at a temperature of 55-60° C. with a current density of 4.7·10⁻⁴ A/cm². The reduction was complete after 4.0 F/mol with a quantitative current efficiency.

EXAMPLE 9 Reduction of Indigo

In the cell according to the invention, comprising a platinum-coated titanium electrode and a cathode consisting of six stainless steel expanded metal lattices, having a total electrode area of 1120 cm², the reduction of indigo to leuco-indigo is carried out.

The anode space and cathode space are separated by cation exchange membrane (Nafion® 424). The divided cell is introduced into a two-cycle electrolysis apparatus having a pump circulation. The flow rate within the cell should be 0.1 m/s. 3000 g of water with 230 g of 50% sodium hydroxide solution are used as the anolyte. The catholyte was prepared by introducing 48.5 g of indigo, calculated as 100%, in a mixture consisting of 5000 g of water, 85 g of EC mediator VE PEDF 120 from DyStar Textilfarben GmbH and 80 g of 50% sodium hydroxide solution into the cathode circulation. A cell voltage of from 1.8 V to 2.0 V and a current of 1 A are applied for the electrolysis. The electrolysis was carried out at a temperature of 55-60° C. with a current density of 8.9·10⁻⁴ A/cm². The reduction was complete after 2 F/mol with quantitative current efficiency.

EXAMPLE 10 Reduction of Indigo

In the cell according to the invention, comprising a platinum-coated titanium electrode and a sandwich cathode consisting of two stainless steel expanded metal layers and a knitted wire fabric layer situated inbetween, having a total electrode area of 2260 cm², the reduction of indigo to leuco-indigo is carried out. The anode space and cathode space are separated by a cation exchange membrane (Nafion® 424). The divided cell is introduced into a two-cycle electrolysis apparatus having a pump circulation. The flow rate within the cell should be 0.1 m/s. 3000 g of water with 230 g of 50% sodium hydroxide solution were used as the anolyte. The catholyte was prepared by introducing 48.5 g of indigo, calculated as 100%, in a mixture consisting of 5000 g of water and 80 g of 50% sodium hydroxide solution into the cathode circulation. For the electrolysis, a cell voltage of 1.8 V and a current of 1 A are applied. The electrolysis was carried out at a temperature of 55-60° C. with a current density of 0.442·10⁻⁴ A/cm². The reduction was complete after 2.5 F/mol with a current efficiency of 80%.

EXAMPLE 11 Metal Recovery

A monopolar divided experimental laboratory cell set up according to the invention contained a multilayer expanded metal cathode consisting of 4 stainless steel expanded metal layers having a base area of 70 cm² each and a stainless steel base cathode plate. The expanded metals had a mesh length of 16 mm and a mesh width of 8 mm with an expanded metal thickness of 1.5 mm. Compared with the two-dimensional projection, the actual cathode surface area was increased by a factor of 2 (surface factor 2). For the total effective cathode surface area, this resulted in an enlargement factor of 4×2+1=9, corresponding to an effective cathode surface area of 70×9=630 cm². A platinum-coated titanium electrode served as the anode, and the membrane consisted of Nafion® 450. A copper sulfate/sulfuric acid solution which contained about 5 g/l of copper and was circulated via the cathode space by pumping was electrolyzed. The anolyte, which was likewise circulated, consisted of a dilute sulfuric acid. The electrolysis current was set at 7 A, corresponding to a current density of 0.1 A/cm² (based on the two-dimensional projection). The actual mean current density was 11 mA/cm². Electrolysis was effected up to a final copper concentration of about 1 g/l.

In an experimental series A, the expanded metals were brought into direct contact with one another. Internal resistance zones between the cathode baseplate provided with the current supply and the individual expanded metal layers were formed only by the contact resistances between the layers. In an experimental series B, the internal resistance zones were enlarged by intermediate strips of graphitized carbon nonwovens (about 1 mm thick). The resulting cell voltage was 4.5 and 4.8 V, respectively. After the end of the experiment, the individual expanded metal layers were removed and the copper deposition was determined by the weight increase and expressed as a ratio of the total amount of copper deposited. The table below shows the percentage current distribution between the individual layers, determined in this manner.

No. of the expanded metal layers Percentage current (considered from distribution over the layers membrane) Experimental series A Experimental series B Layer 1 41 27 Layer 2 27 26 Layer 3 18 24 Layer 4 + cathode 14 23 baseplate

It is clear that it is possible to reduce the decrease in current density from layer to layer by a targeted increase in the size of the internal resistance zones. In spite of the still relatively high current density for the metal deposition, particularly at the front of the first layer, the copper deposition was still sufficiently compact. Furthermore, the expanded metal backs were still well covered with copper.

EXAMPLE 12 Decolorization of a Dye Mixture

The divided experimental laboratory cell according to example 11 was equipped with a multilayer expanded metal anode consisting of a diamond-coated niobium anode baseplate and four niobium expanded metal layers coated on both sides with diamond and having a base area of 100×70 mm (two-dimensional projection 70 cm²). The cathode consisted of stainless steel, and the cation exchange membrane of Nafion® 450.

The anolyte used was a dye solution which was to be decolorized and contained 1 g/l of a dye mixture and to which small amounts of sodium sulfate (about 8 g/l) were added to improve the conductivity. The dye mixture consisted of 1 part of black (C.I. Reactive Black 5), 1 part of blue (C.I. Reactive Blue 21), 1 part of red (C.I. Reactive Red 128) and 0.1 part of yellow (C.I. Reactive Orange 96). Electrolysis was effected with a current of 7 A and a current density of 0.1 A/cm², based on the cross-sectional area of the multilayer expanded metal anode. The surface factor was about 8, so that an actual mean current density of 12.5 mA/cm₂ resulted. The table below shows the course of the decolorization as a function of the specific current input.

The resulting cell voltage was about 5.8 V. The anodically decolorized solution, whose pH had fallen from 11 to about 1.2, was used as the catholyte in the next cycle. The pH increased to 10.5 again.

Color component Color Color Weighted Spec. Yellow, component component color Electrolysis current 436 nm Red, 525 nm Blue, 620 nm characteristic time input Extinct. Extinct. Extinct. Extinct. in in in Decolor. in Decolor. in Decolor. in Decolor. min Ah/l m⁻¹ In % m⁻¹ in % m⁻¹ in % m⁻¹ in % Starting 0 0.00 435 0.0 1069 0.0 1386 0.0 1126 0.0 solution After 15 1.75 190 56.3 272 74.6 189 86.4 224 80.1 electrolysis 30 3.50 37 91.5 16.3 98.5 5.3 99.6 28.4 97.5 at 45 5.25 3.4 99.2 1.7 99.8 1.1 99.9 2.5 99.8 i = 0.1 A/cm² 60 7.00 1.0 99.8 0.6 99.9 0.3 100 0.8 99.9

EXAMPLE 13 Anodic Pollutant Degradation

The divided experimental laboratory cell from example 11 was equipped with a multilayer expanded metal anode consisting of four titanium expanded metal electrodes coated with platinum on both sides and measuring 100×70 mm, and brought into contact with a likewise platinum-coated titanium anode baseplate. The cathode consisted of stainless steel. The anolyte consisted of 1 liter of a solution which was circulated via the anode space by pumping and contained 20 g/l of sulfuric acid and, as the pollutant to be anodically degraded, 0.1 g/l of dichlorophenol. The catholyte used was a sulfuric acid containing 20 g/l. Electrolysis was effected with a current of 35 A, corresponding to a current density, based on the two-dimensional projection of 0.5 A/cm². The enlargement factor of the anode surface area was 8.3, so that a mean current density of 60 mA cm² resulted. The cell voltage was 5.6 V. The results obtained as a function of the electrolysis time are listed in the table below.

Electrolysis time Spec. current DClPh content in DClPh in min input in Ah/l mg/l degradation in % 0 0 100 0 10 5.8 29 71 20 11.6 8 92 30 17.5 2 98 40 23.2 0.5 99.5

As is evident from the table, as much as 98% of the dichlorophenol had been degraded after an electrolysis time of 30 min, corresponding to a current input of 17.5 Ah/l.

EXAMPLE 14 Reduction of Indigo

For the reduction of indigo to leuco-indigo, electrolysis was effected in the divided experimental laboratory cell according to the invention, having a platinum-coated titanium electrode and a multilayer expanded metal cathode of stainless steel. Anode space and cathode space were separated by a cation exchange membrane (Nafion® 424). Two different cathode versions were used. In experiments 1 to 3, the multilayer expanded metal cathode consisted of the cathode baseplate of stainless steel, of two stainless steel expanded metal layers and of a knitted wire intermediate layer located inbetween (sandwich electrode). The individual layers had a base area of 100×100 mm=100 cm². The surface factor was 21.3, so that an effective total cathode area of 2130 cm² resulted. In experiment 4, 6 expanded metal electrodes were brought into contact with one another and with the cathode baseplate and having a base area of 100 cm² each were used. The surface factor was 11.2, so that a total surface area of 1120 cm² resulted. In all four experiments, the anolyte used was a solution of 230 g of 50% strength sodium hydroxide solution in 3000 g of water. The catholyte was prepared by introducing 48.5 g of indigo (calculated as 100%) into a mixture consisting of 5000 g of water and 80 g of 50% sodium hydroxide solution and different amounts of the mediator VE PEDF 120® from DyStar.

The divided cell was introduced into a two-cycle electrolysis apparatus having a pump circulation. The flow rate of the catholyte inside the cell was regulated to 0.1 m/s. The electrolysis was carried out at a temperature of from 55 to 60° C. The reduction was terminated after a current input of 2 F/mol, and the current efficiency of the indigo reduction was determined. The differing experimental data and the results are listed in the table below.

Mediator Current Cell Current Exp. in Current density in voltage efficiency No. Cathode g/batch in A mA/cm² in V in % 1 Sandwich 85 1 0.47 1.8 100 2 Sandwich 120  2 0.94 2.1 100 3 Sandwich none 1 0.47 1.8 80 4 6 layers 85 1 0.89 1.9 100 of expanded metal

EXAMPLE 15 Reduction of C.I. Sulfur Black 1

In the experimental laboratory cell according to the invention of example 14, comprising a platinum-coated titanium electrode, a multilayer expanded metal cathode consisting of a cathode baseplate of nickel and three nickel expanded metal layers having an effective total area of 1050 cm² (surface factor 10.5) was used. Once again, Nafion® 424 was used as the cation exchange membrane. Sulfur Black 1 was reduced to leuco sulfur black 1. For this purpose, the cell was once again introduced into a two-cycle electrolysis apparatus having a pump circulation. The flow rate in the cathode space was 0.1 m/s.

3000 g of water with 234.75 g of 50% sodium hydroxide solution were used as the anolyte, to which a further 518 g of 50% sodium hydroxide solution were added gradually in the course of the electrolysis. The catholyte was prepared by introducing 100 g of C.I. Sulfur Black 1 press cake from DyStar into a mixture consisting of 2000 g of water and 12 g of 50% sodium hydroxide solution. A further 4350 g of the press cake were added in the course of the electrolysis.

The electrolysis was effected on the basis of EP 101 222 10 B1 at a current of 10 A (current density: 9.5 mA/cm²), a cell voltage of from 7.0 to 4.3 V resulting. The reduction was complete after introduction of a charge quantity of 238.3 Ah. 4350 ml of a leuco sulfur black 1 solution having a concentration of reducing agent equivalents of 438 Ah, based on 1 kg of dry dye, were obtained.

EXAMPLE 16 Reduction of Nitrobenzene

In the cell according to the invention, according to example 15, comprising the multilayer nickel expanded metal cathode, nitrobenzene was reduced to azobenzene. The flow rate in the cathode space was once again set at 0.1 m/s. 1000 ml of 70% strength ethanol with 25 g of sodium acetate and 100 g of nitrobenzene were used as the catholyte. 2000 ml of a concentrated sodium carbonate solution were used as the anolyte. The electrolysis was carried out on the basis of Zeitschrift für Elektrochemie [Journal for Electrochemistry] 1898; 5; pages 108-113, using a current of 10 A (total current density 9.5 mA/cm²). The reduction was complete after a charge quantity of 87.5 Ah. The resulting yellow alcoholic solution was removed from the cathode circulation, the excess ethanol was removed in vacuo, and the residue was taken up in water and extracted several times with ethyl acetate. The combined organic phases were dried over sodium sulfate, and the excess solvent was distilled off. 63 g of azobenzene (85% of theory) were obtained with a quantitative current efficiency.

EXAMPLE 17 Reduction of C.I. Vat Yellow 46

In the cell according to the invention, comprising a platinum-coated titanium electrode and a sandwich cathode consisting of two stainless steel expanded metal layers and a knitted wire intermediate layer located inbetween, having a total electrode area of 2130 cm², the reduction of C.I. Vat Yellow 46 to its leuco compound was carried out. The anode space and cathode space are separated by a cation exchange membrane (Nafion® 424). The divided cell is introduced into a two-cycle electrolysis apparatus having a pump circulation. The flow rate within the cell was adjusted to 0.1 m/s.

3000 g of water with 230 g of 50% sodium hydroxide solution were used as the anolyte. The catholyte was prepared by introducing 48.6 g of C.I. Vat Yellow 46, calculated as 100% (dried press cake obtained from production), in a mixture consisting of 5000 g of water, 171 g of EC mediator VE PEDF 120® from DyStar and 80 g of 50% sodium hydroxide solution, into the cathode circulation. The electrolysis current was adjusted to 2 A, a cell voltage of from 2.0 to 2.2 V resulting. The electrolysis was carried out at a temperature of 55-60° C. with a current density of 0.94 mA/cm², and was complete after a current input of 4.61 F/mol with a current efficiency of 87%.

EXAMPLE 18 Reduction of C.I. Vat Green 1

In the cell according to the invention, as in example 17, the reduction of C.I. Vat Green 1 to its leuco compound was carried out.

The flow rate within the cell was 0.1 m/s. 3000 g of water with 230 g of 50% strength sodium hydroxide solution were used as the anolyte. The catholyte was prepared by introducing 50 g of C.I. Vat Green, calculated as 100% (dried press cake obtained from production), in a mixture consisting of 5000 g of water, 171 g of EC mediator VE PEDF 120® from DyStar and 80 g of 50% sodium hydroxide solution, into the cathode circulation. For the electrolysis, a cell voltage of from 1.6 to 2.0 V and a current of 0.5 A were applied. The electrolysis was carried out at a temperature of 55-60° C. with a current density of 0.23 mA/cm². The reduction was complete after 2.08 F/mol with quantitative current efficiency.

EXAMPLE 19 Reduction of C.I. Vat Red 10

In the same cell and apparatus as in example 17, the reduction of C.I. Vat Red 10 to its leuco compound was carried out. The flow rate within the cell was once again adjusted to 0.1 m/s. 3000 g of water with 230 g of 50% sodium hydroxide solution were used as the anolyte. The catholyte was prepared by introducing 50 g of C.I. Vat Red 10, calculated as 100% strength (dried press cake obtained from production), in a mixture consisting of 5000 g of water, 171 g of EC mediator VE PEDF 120® from DyStar and 80 g of 50% sodium hydroxide solution and filling the resulting solution into the cathode circulation. For the electrolysis, a cell voltage of from 2.4 to 2.8 V and a current of 2 A were applied. The electrolysis was carried out at a temperature of 55-60° C. with a current density of 0.94 mA/cm². The reduction was complete after a current input of 4.51 F/mol with a current efficiency of 88.7%.

EXAMPLE 20 Reduction of C.I. Vat Blue 6

In the same cell and circulation apparatus as in example 18, the reduction of C.I. Vat Blue 6 to its leuco compound was carried out. The flow rate within the cell was adjusted to 0.1 m/s.

3000 g of water with 230 g of 50% sodium hydroxide solution were used as the anolyte. The catholyte was prepared by introducing 50 g of C.I. Vat Blue 6, calculated as 100% strength (dried press cake obtained from production), in a mixture consisting of 5000 g of water, 171 g of EC mediator VE PEDF 120® from DyStar and 80 g of 50% sodium hydroxide solution. For the electrolysis, a current of 1 A was applied, a cell voltage of about 2.0 V resulting. The electrolysis was carried out at a temperature of 55-60° C. with a current density of 0.47 mA/cm². The reduction was complete after 4.0 F/mol with a quantitative current efficiency.

EXAMPLE 21 Vat Dyes

The experimental laboratory cell according to the invention, comprising the platinum-coated titanium anode and the Nafion® 424 cation exchange membrane, was equipped with a multilayer expanded metal cathode consisting of 12 stainless steel expanded metal layers of 100 cm² each. The surface factor was 25, and the effective total area was 2500 cm². The cell was integrated into the two-cycle circulation apparatus. The catholyte consisted of 3 l of a solution having the following composition:

-   -   1 g/l of C.I. Vat Blue 6     -   1.62 g/l of FeCl₂     -   13.56 of triethanolamine     -   12.48 of NaOH

A 0.1 N NaOH served as the anolyte. Electrolysis was effected at a temperature of 28° C. with a current of 2.5 A, corresponding to an average current density of 1 mA/cm². Under these conditions, a current efficiency of 89% was achieved. By coupling such a reduction cell with a dye plant, and using potential control, electrochemical dyeing can be carried out while dispensing with the customary addition of dithionite for dye reduction.

EXAMPLE 22 Cathodic Decolorization of C.I. Reactive Red 4

In the electrolysis cell of example 21, comprising 12 stainless steel expanded metal layers, a dye solution of 0.5 g/l of C.I. Reactive Red 4 was cathodically treated. 2 l of catholyte solution having an NaOH content of 0.6 mol/l were used. A sodium hydroxide solution having a content of 5 g/l served as the anolyte. Electrolysis was effected with a current of 2 A, corresponding to an average current density of 0.8 mA/cm². The resulting cell voltage was 5.9 V, at an electrolysis temperature of about 30° C. After a specific current input of 4 Ah/l, the solution had been 81% decolorized (based on weighted color characteristics, cf. example 12).

The following reference numerals are used in the FIGS. 1 to 7:

1 Anode plate

2 Cathode baseplate

3 Electrode base body

4 Cathode frame

5 Cooling ducts

6 Anolyte inlet

7 Anolyte outlet

8 Catholyte inlet

9 Catholyte outlet

10 Cooling medium inlet

11 Cooling medium outlet

12 Expanded metal layers

13 Porous intermediate layers

14 Anode sealing frame

15 Ion exchange membranes

16 Anode spacer

17 Cathode spacer

18 Contact rails

19 Cathode baseplate with expanded metal inlay

20 Anode space

21 Entrance of separate catholyte

22 Exit of separate catholyte

A Electrolysis cell comprising two separate expanded metal segments

B Catholyte circulation 1 for segment of higher current density

C Catholyte circulation 2 for segment of lower current density

D Catholyte metering station

E Catholyte outlet

F Gas outlet (catholyte)

G Anolyte metering station

H Anolyte gas separator

I Anolyte outlet

J Gas outlet (anolyte) 

1. An electrolysis cell comprising sheet-like anodes and cathodes which are separated from one another by means of separators and which are arranged in a cell trough or in a plurality of electrode frames clamped to one another and are connected in an electrically monopolar or bipolar manner, wherein the cathodes and/or anodes are in the form of multilayer expanded metal electrodes which consist of at least two expanded metal layers which are in contact with one another via internal resistance zones and through which the electrolyte solutions flow in the longitudinal direction.
 2. The electrolysis cell as claimed in claim 1, wherein one or more intermediate layers of a porous electrode material are arranged between in each case at least two of the expanded metal layers through which flow takes place longitudinally and are in contact with said expanded metal layers.
 3. The electrolysis cell as claimed in claim 1, wherein the multilayer expanded metal electrode consisting of a plurality of expanded metal layers and optionally of porous intermediate layers has a specific electrode surface area which is from 5 to 100 times greater compared with a plate electrode.
 4. The electrolysis cell as claimed in claim 1, wherein the internal resistance zones are formed by the contact resistances between the expanded metal layers and/or between the expanded metal layers and the porous intermediate layers.
 5. The electrolysis cell as claimed in claim 1, wherein individual or a plurality of expanded metal layers and/or intermediate layers are separated from one another by spacers of electrically nonconductive material and are connected to one another in an electrically conductive manner only via laterally arranged contact surfaces, the current flowing through the multilayer expanded metal electrode in a meandering manner.
 6. The electrolysis cell as claimed in claim 1, wherein the multilayer expanded metal electrode packet is pressed, optionally with intermediate layers of porous material, by pressure means onto an electrode baseplate connected to the current supply, and contact is thus produced between the individual expanded metal layers and between said layers and the electrode baseplate.
 7. The electrolysis cell as claimed in claim 1, wherein a plurality of expanded metal segments separated by porous intermediate layers are provided with different current densities by separate supply lines and discharge lines for the electrolyte solutions, and wherein the electrolyte solution flows first through the segment closest to the opposite electrode and having the higher current density and then through the segment further away from the opposite electrode and having the lower current density.
 8. The electrolysis cell as claimed in claim 7, wherein the electrolyte flows in said separated expanded metal segments are hydrodynamically coupled to one another via the porous intermediate layer.
 9. The electrolysis cell as claimed in claim 1, wherein the expanded metal layers consist of stainless steel, nickel or copper or of valve metals coated by means of noble metals, noble metal oxides or doped diamond.
 10. The electrolysis cell as claimed in claim 2, wherein the individual expanded metal layers, the plastic spacers and optionally the intermediate layers of the porous electrode material have a thickness of from 1 to 5 mm and a void volume of at least 70%.
 11. The electrolysis cell as claimed in claim 2, wherein the porous intermediate layers consist of knitted metal wire fabrics, metal foams or nonwoven carbon fiber fabrics.
 12. The electrolysis cell as claimed in claim 1, wherein the multilayer expanded metal electrode is formed from an expanded metal web which is wound in the form of a spiral and is in contact with an outer or inner tube of a cylindrical hydrolysis cell, which tube is provided with the current supply.
 13. The electrolysis cell as claimed in claim 1, wherein, instead of individual or a plurality of expanded metal layers, expanded metal layers comprising woven metal wire fabrics are used. 14-18. (canceled)
 19. A process for complete or partial cathodic reduction of organic and/or inorganic compounds which comprises using of the electrolysis cell as claimed in claim
 1. 20. The process as claimed in claim 19, wherein dyes in the form of dye solutions or solutions in dyeing works are cathodically reduced or wastewaters in dyeing works are decolorized.
 21. A process for the complete or partial anodic oxidation of organic and/or inorganic compounds which comprises using of the electrolysis cell as claimed in claim
 1. 22. A process for oxidative degradation of pollutants in process solutions and wastewaters which comprises using the electrolysis cell as claimed in claim
 1. 23. A process for removing metals from process solutions and wastewaters which comprises using the electrolysis cell as claimed in claim
 1. 